Pressure-sensitive adhesives (sometimes referred to as PSA) which are permanently tacky in dry form at room temperature are widely used for making labels and tapes which can be applied to a variety of substrates and adhere on application of slight pressure. They are also used for laminating polymeric films such as poly(vinyl chloride) and polyester Mylar, silicone coated papers, and film release liners for forming decals and other related products.
Water based pressure-sensitive adhesives are of interest because of their low VOC emissions. Although the performance is not equivalent to solvent based pressure-sensitive adhesives, they satisfy emission standards and are easy to process. Common types of pressure-sensitive adhesives, both water based and solvent based, are derived from acrylic ester based copolymers, such as alkyl acrylate and alkyl methacrylate copolymers.
The following patents and articles are representative of acrylic based pressure-sensitive adhesives:
Hidalgo, et al. xe2x80x9cPolystyrene(1)/poly(butyl acrylate-methacrylic acid)(2) core-shell emulsion polymers. Part II: Thermomechanical properties of latex films,xe2x80x9d Colloid and Polymer Science, 1992, Vol. 270, pages 1208-1221, disclose the formation of polystyrene/poly(butyl acrylate-methacrylic acid) latexes by a two stage process. Initially, a polystyrene seed is prepared and then the butyl acrylate and methacrylic acid polymerized in the presence of the seed forming a core/shell polymer in a ratio of 2/3.
EP 0 593231 A1 discloses the formation of pressure-sensitive acrylic adhesives by the addition of low molecular weight ( less than 7,000) ethylene oxide-block-propylene oxide copolymer surfactants to acrylic pressure-sensitive adhesives for the purpose of improving low temperature adhesion. These pressure-sensitive adhesives are based upon 2-ethylhexyl acrylate and acrylic and methacrylic esters of C4-12 alkanols, such as butyl acrylate.
U.S. Pat. No. 6,225,401 discloses filterable aqueous dispersions of pressure-sensitive adhesive suited for labels formed by copolymerizing acrylic or methacrylic esters in the presence of an inhibitor. A wide variety of hardening comonomers can be included in the emulsion polymerization process and these include the styrenes, acrylonitrile, vinyl esters, and so forth.
U.S. Pat. No. 6,254,985 discloses aqueous emulsions of pressure-sensitive adhesives base upon esters of acrylic and methacrylic acid. The patentees disclose the use of an emulsifier consisting of at least 5% by weight of aromatic carbon atoms, typically including at least two sulfonate groups to improve adhesion and cohesion.
The present invention is directed to an improvement in a process for preparing an aqueous emulsion of a pressure-sensitive adhesive based upon acrylic esters which have a good balance of adhesive and cohesive properties and to the resulting emulsion. In the basic process, a pressure-sensitive adhesive formulation comprised of at least one ester of acrylic or methacrylic acid is polymerized in the presence of water and an emulsifier thereby forming an emulsion polymerized pressure-sensitive adhesive polymer. The improvement resides in effecting the polymerization of said pressure-sensitive adhesive formulation comprised of an ester of acrylic or methacrylic acid and a styrene containing polymer containing at least 80 percent by weight styrene, said styrene containing polymer present in an amount of from 5 to 30 percent by weight of the pressure-sensitive adhesive formulation.
Significant advantages of the process and product can be realized and they include:
an ability to include a small proportion of a low cost filler polymer into an aqueous pressure-sensitive adhesive based upon acrylic and methacrylic esters without adversely affecting the adhesive properties;
an ability to overcome deficiencies in film formation from blends of polymers, e.g., blends of polystyrene and acrylic and methacrylic copolymers;
an ability to include a low cost xe2x80x9cfillerxe2x80x9d into a pressure-sensitive adhesive by a simple method without the need for special equipment;
an ability to prepare a pressure-sensitive adhesive with high Tg polystyrene filler in one reaction, and in a single reactor, while maintaining the performance advantages of the pressure-sensitive adhesive including those formed by the blend method; and,
an ability to eliminate the need for the high Tg polystyrene latex to be of a specific particle size, expensive macromers, and minimum amounts of surfactants.
Emulsion polymerization of a pressure-sensitive adhesive formulation comprised of esters of acrylic and methacrylic acid including ethylenically unsaturated monomers to produce aqueous based pressure-sensitive adhesive polymer emulsions is well known. A representative pressure-sensitive adhesive formulation is comprised of an aqueous polymer dispersion wherein the polymer is comprised of polymerized units, based on the total weight of units, of (a) from 60 to 95% by weight of at least one C6-12 alkyl acrylate; (b) from 0 to 10% by weight of an ethylenically unsaturated compound having a glass transition temperature of above 0xc2x0 C. and contain no functional groups other than ethylenically unsaturated group; (c) from 0 to 10% by weight of an ethylenically unsaturated compound having at least one acid or acid anhydride group; and (d) from 0 to 20% by weight of a further ethylenically unsaturated compound; the weight percentages based on the total weight of polymer. Typically, unsaturated compounds in group (c) and (d) are included at less than 5% each, when used. Thus, compounds in group (c) and (d) comprise a small proportion of the pressure-sensitive adhesive.
Particularly suitable alkyl acrylates in group (a) are 2-ethylhexyl acrylate, octyl acrylate, decyl acrylate or dodecyl acrylate.
Monomers in group (b) can include methyl methacrylate, methyl acrylate, n-butyl acrylate and tert-butyl acrylate; vinyl esters of C1-20 carboxylic acids such as vinyl laurate, stearate, propionate, the vinyl ester of Versatic acid, and vinyl acetate; vinyl aromatics such as styrene, and so forth. Methyl methacrylate is preferred.
Examples of group (c) monomers can include acrylic and methacrylic acid, maleic acid, or maleic anhydride. Group (d) monomers can include C1 to C10 hydroxyalkyl (meth)acrylates.
There are two mechanisms in the process for forming the pressure-sensitive adhesive including the styrene containing polymer, e.g., polystyrene filler. In one mechanism, a styrene containing polymer is dissolved in a pressure-sensitive adhesive formulation comprised of a mixture of monomers. The resulting solution, then, is emulsified with surfactants and water and, with the aid of energy supplied by high shear mixing, converted to a stable emulsion of relatively small particle size particles. The resulting emulsion is polymerized by emulsion polymerization. In a second method, a seed latex of styrene polymer is prepared by emulsion polymerization and the pressure-sensitive adhesive formulation emulsion polymerized in the presence of the seed latex. The second method has the advantage of allowing for reduced emulsifier in the final product, elimination of high shear mixing and allowing the reaction to be carried out in a single reactor.
The styrene containing polymer is one containing at least 80% by weight styrene and typically one containing 100% styrene by weight. Optional monomers that may be included in producing styrene containing polymers are xcex1-methyl styrene, xcfx81-methylstyrene, acrylonitrile, methacrylonitrile, methacrylonitrile, methyl methacrylate and trace levels of other monomers leading to a high Tg polymer, at least 80xc2x0 C.
The styrene copolymer is incorporated into the pressure-sensitive adhesive in an amount from 5 to 30% by weight, which includes the base pressure-sensitive adhesive polymer and the styrene containing polymer. Levels above about 30% by weight detract from the performance of the pressure-sensitive adhesive. Levels below about 5% by weight, although not adversely affecting the properties of the pressure-sensitive adhesive, do not afford the low cost advantages. Preferably, the level of styrene copolymer is from 10 to 20% by weight.
Many of the pressure-sensitive adhesive formulations include styrene as a comonomer. Usually, styrene is included at low levels, e.g., below 5% by weight. High levels of styrene in the pressure-sensitive adhesive formulation lead to unacceptably high Tg of the pressure-sensitive adhesive polymer. Such high levels of styrene adversely affect loop tack in the PSA. Accordingly, the ability to incorporate an amount of styrene via copolymerization into the pressure-sensitive adhesive polymer equal to that where the pressure-sensitive adhesive formulation is polymerized in the presence of styrene containing polymer as in the hybrid composite is not an acceptable option.
The pressure-sensitive adhesive formulation is designed to lead to a hybrid composite having a Tg of xe2x88x9225xc2x0 C. to xe2x88x9290xc2x0 C., preferably xe2x88x9240xc2x0 C. to xe2x88x9275xc2x0 C. and a loop tack adhesion value greater than 1 pound per linear inch (pli); preferably greater than 1.5 pli, according to Pressure-sensitive Test Council (PSTC) test method PSTC-5, tested on stainless steel panel. Even though the Tg is not as well defined as in a single polymer, the Tg of the composite affords an approximation.
Polymerization of the seed latex, as well as the pressure-sensitive adhesive formulation can be initiated by thermal initiators or by a redox system. A thermal initiator is typically used at temperatures at or above about 70xc2x0 C. and redox systems are preferred at temperatures below about 70xc2x0 C. The amount of thermal initiator used in the process is 0.1 to 3 wt %, preferably not more than about 0.5 wt %, based on total monomers. Thermal initiators are well known in the emulsion polymer art and include, for example, ammonium persulfate, sodium persulfate, and the like. The amount of oxidizing and reducing agent in the redox system is about 0.1 to 3 wt %. Any suitable redox system known in the art can be used; for example, the reducing agent can be a bisulfite, a sulfoxylate, ascorbic acid, erythorbic acid, and the like. The oxidizing agent can include hydrogen peroxide, organic peroxide such as t-butyl peroxide, persulfates, and the like.
Chain transfer agents, well known in the aqueous emulsion polymerization art; are typically used but are not required. Examples include dodecyl mercaptan, mercaptocarboxylic acids, and esters of mercaptocarboxylic acid. The chain transfer agent is added at levels of about 0.01 to 0.5 wt %, preferably 0.02 to 0.15 wt %, based on the weight of monomers.
Effective emulsion polymerization reaction temperatures range from about 50 to about 100xc2x0 C. depending on whether the initiator is a thermal or redox system.
The emulsifying agents which are suitably used are typically anionic, nonionic or blends thereof. Suitable nonionic emulsifying agents include polyoxyethylene condensates. Polyoxyethylene condensates may be represented by the general formula: Rxe2x80x94(CH2CH2Oxe2x80x94)nH where R is the residue of a fatty alcohol containing 10 to 18 carbon atoms, an alkylphenol, a fatty acid containing 10 to 18 carbon atoms, an amide, an amine, or a mercaptan, and where n is an integer of 1 or above. The Igepal surfactants are members of a series of alkylphenoxy-poly(ethyleneoxy)ethanols having alkyl groups containing from about 7-18 carbon atoms, and having from about 4 to 100 ethyleneoxy units, such as the octylphenoxy poly(ethyleneoxy)ethanols, nonylphenoxy poly(ethyleneoxy)ethanols, and dodecylphenoxy poly(ethyleneoxy)ethanols. Examples of nonionic surfactants include polyoxyalkylene derivatives of hexitol (including sorbitans, sorbides, mannitans, and mannides) anhydride, partial long-chain fatty acid esters, such as polyoxyalkylene derivatives of sorbitan monolaurate, sorbitan monopalmitate, sorbitan monostearate, sorbitan tristearate, sorbitan monooleate and sorbitan trioleate.
Suitable anionic emulsifying agents include the monovalent salts of the sulfates of the above mentioned nonionics, mono or disodium salts of sulfosuccinates half esters or diesters, sodium salts of alkylbenzene sulfonates. A single emulsifying agent can be used, or the emulsifying agents can be used in combination. When combinations of emulsifying agents are used, it is advantageous to use a relatively hydrophobic emulsifying agent in combination with a relatively hydrophilic agent. A relatively hydrophobic agent is one having a cloud point in 1% aqueous solution below 190xc2x0 F. (88xc2x0 C.) and a relatively hydrophilic agent is one having a cloud point in 1% aqueous solution of 190xc2x0 F. (88xc2x0 C.) or above. The concentration range of the total amount of emulsifying agents useful is from 0.5 to 5% based on the aqueous phase of the latex regardless of the solids content. The surfactant package is typically used in an amount of from 2 to 7 wt % of the emulsions.
An alkaline buffering agent of any convenient type that is compatible with the stabilizing agent may be used if it is desired to maintain the pH of the system at a desired value. The amount of buffer is generally about 0.1 to 0.5 wt % based on the monomers.